Arrangement for igniting a gas and/or vapour discharge lamp provided with preheatable electrodes

ABSTRACT

An electronic starter for igniting low-pressure mercury vapour discharge lamps. To prevent local blackening of the lamp after a certain number of switch-on operations, relatively high voltage peaks across the lamp are allowed during the ignition procedure only over a small part per half cycle of the mains voltage. The starter is shortcircuited during the remaining parts of these half cycles in the ignition procedure.

United States Patent [191 Remery et a1.

[451 Nov. 11, 1975 1 1 ARRANGEMENT FOR IGNITING A GAS AND/OR VAPOUR DISCHARGE LAMP PROVIDED WITH PREHEATABLE ELECTRODES [75] lnventors: Michel Remery; Andre Souvay, both of Evreux, France [73] Assignee: U.S. Philips Corporation, New

York, NY.

22 Filed: Mar. 27, 1974 211 Appl. No.: 455,069

[30] Foreign Application Priority Data Mar. 30, 1973 France 73.11530 [52] US. Cl. 315/101; 315/98; 315/105;

315/106; 315/208; 315/D1G. 2; 3l5/DIG. 5 [51] Int. Cl. H05b 41/18 [58] Field of Search 315/94, 98, 101. l05l07,

315/200 R. 202, 205, 207, 208, DIG. 2, DIG. 5

[56] References Cited UNITED STATES PATENTS Koyama et a1 315/101 X 7/1972 Shimizu et a]. 315/200 R 3.705.329 12/1972 Vogeli 315/205 X 3.758.818 9/1973 Kanecla 315/207 Primary E.\'uminer-R. V. Roliinec Assistant E\'zlminerE. R. La Roche Attorney, Agent, or Firm-Frank R. Trifari; Bernard Franzblau [57] ABSTRACT An electronic starter for igniting low-pressure mercury vapour discharge lamps.

To prevent local blackening of the lamp after a certain number of switch-on operations, relatively high voltage peaks across the lamp are allowed during the ignition procedure only over a small part per half cycle of the mains voltage. The starter is short-circuited during the remaining parts of these half cycles in the ignition procedure.

13 Claims, 3 Drawing Figures US. Patent. Nov. 11, 1975 Fig.3

ARRANGEMENT FOR IGNITING A GAS AND/R VAPOUR DISCHARGE LAMP PROVIDED WITH PREHEATABLE ELECTRODES The invention relates to an arrangement for igniting and supplying a gas and/or vapour discharge lamp provided with preheatable electrodes, which arrangement has two input terminals adapted to be connected to an alternating voltage source. Across the input terminals are connected, in the operating condition of the arrangement, a series arrangement of a stabilizing inductor ballast and the lamp. The ends of the preheatable electodes remote from the alternating voltage source are connected by means of a transistor forming part of a starter. The base of the transistor is connected to a control circuit rendering the transistor alternately conducting and non-conducting at a high frequency during the ignition procedure of the lamp.

Arrangements of the kind described (Electronic starters for discharge lamps) have been proposed in US. Pat. 3,875,459, issued Apr. 1, 1975. In arrangements according to this U.S. Pat. the current through the lamp electrodes and the inductor of the stabilizing ballast is repeatedly interrupted at a frequency of, for example, 20 kHz during the ignition procedure of the lamp. As a result relatively high voltage peaks are produced between the lamp electrodes which enhance the ignition of the lamp.

These electronic starters may replace, for example, the glow discharge starters provided with a bimetal.

A drawback of an arrangement according to the said US. Pat. No. is, however, that in lamps provided with preheatable electrodes, which lamps are generally low pressure mercury vapour discharge lamps provided with a fluorescent coating, the fluorescent material is charged in the vicinity of the electrodes. This change leads, for example, to the appearance of dark rings near the ends of the lamp.

It was found that this phenomenon which may be ascribed to the ion bombardment of the electrodes occured after a period of time which, with a constant number of ignitions was substantially proportional to the time during which the relatively high ignition voltages were present between the lamp electrodes.

The inventors subsequently found that it is advantageous to ignite the lamp with a voltage peak occurring between the lamp electrodes whose duration is as short as possible.

It is to be noted that a starter has also been proposed in which the interruption of the current by the inductor started some time after applying the voltage to the arrangement. The object was to heat the electrodes to temperature favourable for fast ignition. The arrangements according to this proposal allowed several ten thousand ignitions before the lamp quality was notably changed. On the other hand these prior arrangements could not be used instead of glow discharge starters because the operation of these arrangements required a third connection with one of the terminals of the voltage supply, which might be a drawback in some cases.

One of the objects of the invention is to realize a starter having not more than two connections and which allows a large number of lamp ignitions before the quality of teh fluorescent material of the lamp is noticeably changed.

According to the invention an arrangement for igniting and supplying a gas and/or a vapour discharge lamp provided with preheatable electrodes has two input terminals intended to be connected to an alternating voltage source. In the operating condition of the arrangement, the input terminals are connected by a series arrangement of a stabilizing inductor ballast and the lamp. The ends of the preheatable electrodes remote from the alternating voltage source are connected by means of a transistor forming part of a starter. The base of the transistor is connected to a control circuit for rendering the transistor alternately conducting and non-conducting at a high frequency during the ignition procedure of the lamp. The invention is characterized in that the starter is provided with a further electric switching element by which the control of the transistor during the ignition procedure of the lamp is released not longer than during the first half of each half cycle of the electrical supply. The said further electric switching element is incorporated in a branch connected in parallel with the transistor and is provided with an operating member by means of which said switching element is in the conducting condition during the remaining parts of the said half cycles during the ignition procedure.

An advantage of this arrangement is that the period during which a relatively high voltage is present between the lamp electrodes is relatively short.

In a preferred embodiment according to the invention the ends of the preheatable electrodes remote from the alternating voltage source are connected by means of the transistor through a rectifier bridge and in which not only the transistor but also a control circuit having a short time constant is connected to two output terminals of this rectifier bridge. In addition, the base of the transistor is connected through a breakdown element to this control circuit. The further electric switching element is a semiconductor switching element having a first main electrode connected directly to one output terminal of the rectifier bridge and a control electrode connected to said one bridge output terminal at least through a resistor. The other main electrode of the semiconductor switching element is connected either to the other output terminal of the rectifier bridge or to the control circuit of the transistor.

An advantage of this preferred embodiment is its simple structure.

The further electric switching element is preferably a thyristor. An advntage thereof is that the control circuit may be uncomplicated.

When using the said thyristor the fast current interruptions are blocked when this thyristor is rendered conducting until the instant when the current flowing in the thyristor has become zero again. The current through the thyristor also results in the lamp electrodes being preheated during a large part of the period of the ignition process of the lamp. As a result these lamp electrodes will quickly achieve the temperature at which ignition can take place. The peak voltages to be generated with the transistor are applied, for example, during the 0.01to 0.5" part of the duration of the half cycles of the supply and preferably during the 0.05 to 0.2" part of these half cycles. This reduces damage to the lamp which may result from the ion bombardment during the start. In a practical case the said part of the half cycle in which peak voltages were generated was 0.01.

As compared with the prior art starter the arrangement according to the invention may have the additional advantage that under otherwise equal circumstances ignition is effccted with peak voltages having a lower amplitude than those which were generated in the known model. This also contributes to thc reduction of the said ion bombardment effects and allows the use of a normal transistor.

The invention will be further described with reference to the accompanying drawing in which:

FIG. 1 shows the electrical circuit of a first arrangement according to the invention;

FIG. 2 shows the electrical circuit of a second arrangement according to the invention; and

FIG. 3 shows an oscillogram of the voltage between the terminals of the lamp, in an arrangement according to the invention, during the ignition procedure of this lamp.

In FIG. 1 an input terminal 1 of the arrangement is connected through an inductive stabilizing ballast 6 to one end of a preheated electrode 3 of a lowpressure mercury vapor discharge lamp provided with a fluorescent coating. Terminal 2 of the arrangement is directly connected to one end of the other electrode 4 of this lamp 5. The terminals 1 and 2 are intended to be connected to an alternating voltage supply of approximately 220 Volt, 50 Hz.

The other end of the electrode 3 is connected to a terminal 7 while the other end of the electrode 4 is connected to a terminal 8. These terminals 7 and 8 constitute the input terminals of two diode groups 9-10 and 11-12 jointly constituting a rectifier bridge.

An output terminal 13 of this bridge is connected to a conductor 15 of this part of the circuit which represents an electronic starter. An output terminal 14 of the bridge is connected to a second conductor 16 of this starter.

An npn-transistor 17 is present between the conductors 15 and 16, and the collector of this transistor is connected to the positive conductor 15, while the emitter is connected to the negative conductor 16. The base of the transistor 17 is connected through a resistor 18 to the conductor 16. e conductor 16. The base of the transistor 17 is connected through a resistor 18 to the conductor 16.

A circuit having a short time constant is constituted by a resistor 19 in series with a capacitor 20. This series arrangement is provided between the conductors l5 and 16, while the junction of the resistor and the capacitor is connected to the base of the transistor 17 through a breakdown element, namely a disc 21. A resistor 22 having a negative temperature coefficient is connected in parallel with the capacitor 20.

The anode of a thyristor 23 is connected to the positive supply conductor 15 and the cathode of the said thyristor is connected to the negative supply conductor 16. The control electrode of the thyristor 23 is connected to the positive supply conductor 15 through a glow discharge tube 24, containing neon, which is arranged in series with a resistor 25.

The arrangement shown in FIG. 1 operates as follows. When the voltage is applied between the terminals 1 and 2 the lamp 5 is not ignited yet. Substantially the entire supply voltage is then present across the diode bridge 9-11 and 10-12. This bridge then starts to feed the transistor 17 during each half cycle of the supply voltage.

Initially, however, transistor 17 is cutoff and the capacitor is charged through the resistor 19 until the voltage at the terminals of the capacitor 20 has become equal to the threshold voltage of the breakdown element 21. At that instance the transistor 17 starts to conduct and the electrode preheating circuit of the lamp 5 is closed. When the capacitor 20 is discharged the transistor 17 is cut off again and the capacitor 20 is recharged. This procedure is repeated a large number of times at the commencement of the half cycles of the AC supply voltage, which gives rise to voltage peaks between the lamp electrodes with the amplitudes of these peaks increasing to a limited extent (FIG. 3). At the instant when the voltage peak reaches a given threshold V (FIG. 3) thyristor 23 becomes conducting so that the terminals 13 and 14 are short-circuited, which instantaneously terminates the fast interruption of the current through the transistor and results in a preheating of the electrodes 3 and 4 of the lamp 5. The thyristor 23 remains conducting until the instant when the current through this thyristor has again become zero. This procedure is repeated at each successive half cycle of the supply voltage.

When the temperature of the electrodes has increased sufficiently, the lamp is ignited during an interruption phase of the current through the transistor. This situation immediately terminates as a result of the lower operating voltage then occurring across the lamp (FIG. 3).

In a manner as described in the above US. Patent, the task of the NTC resistor 22 is to block the operation of the transistor if the lamp 5 is not ignited within a number of seconds, for example, because this lamp is defective.

By influencing the resistive value of the resistor 25 it is possible to vary the ratio t lt where t is the period of application of the ignition peakvoltages and t corresponds to the duration of a half cycle of the supply voltage. Said ratio may vary for example between H to A and is preferably between H20 and US. It was found by experiment that a ratio t /t in the order of H10 is very satisfactory. This is approximately the ratio which is also shown in FIG. 3. FIG. 3 shows two half cycles preceding a half cycle in which the lamp was actually ignited.

A neon glow discharge tube 24 which is present in a control circuit of the control electrode of the thyristor 23 has for its task to maintain the trigger voltage for the thyristor substantially constant over the entire temperature range in which the arrangement must operate.

In the arrangement according to the invention shown in FIG. 2, the cathode of the thyristor 23 is connected to the base of the transistor 17 through a currentlimiting resistor 26.

The operation of the arrangement according to FIG. 2 differs from the operation of the arrangement according to FIG. 1 in that the ignition of the thyristor 23 leads to a longer conducting period of the transistor 17. In this modification the transistor 17 therefore has a dual function, namely to ensure that the current is interrupted for generating the peak voltages and subsequently to convey the preheating current of the lamp electrodes 3 and 4.

In the arrangement according to FIG. 2 the intensity of the current through the thyristor 23 is low and this is due to the current through the transistor 17. Such a modification may be preferred when a small thyristor is to be used or in the case where the starter is used for igniting a group of lamps whose total electrode preheating current exceeds the maximum current allowed for the thyristor type in the circuit of FIG. 1.

The arrangement according to the invention for igniting the lamp was realized by means of the components mentioned hereinafter:

Lamp 5 low-pressure mercury vapour discharge lamp 40 Watt Ballast 6 l H I Diodes 9 to 12 BYX l Transistor l7 BU I Resistor l8 150 Ohm.

Resistor 19 12 kOhm.

Capacitor 20 1 47 nF.

Diac 21 Br 100.

Resistor 22 2 kOhm at 25C.

Thyristor 23 Bt 109 Neon tube 24 85 V Resistor 25 100 kOhm.

Resistor 26 r 2.2 kOhm.

With these components and for an ambient temperature of approximately 20C the lamp was ignited in approximately /2 second at a supply voltage of 220 Volt, 50 Hz. The lamp thus described allowed approximately 40,000 ignitions before the appearance of the lamp noticeably changed.

It is to be noted that it is feasible to replace, for example, in the circuit of FIG. 1 the resistor 19 by a temperature-dependent resistor, namely an NTC resistor. With this resistor it can be achieved that the voltage peaks between the lamp electrodes 3 and 4, generated with the aid of the transistor 17, are generated only after a certain preheating of the lamp electrodes 3 and 4. An advantage thereof is that the risk of ignition of the lamp 5 on cold electrodes 3 and 4 is reduced. Another advantage is that it now becomes possible to increase the voltage value of the voltage peaks, if desired, for example, to further reduce the ignition time.

Steps other than those with reference to the abovementioned NTC resistor are of course feasible to postpone the generation of the voltage peaks until a certain preheating of the lamp electrodes 3 and 4 through the thyristor 23 has taken place.

What is claimed is:

1. An ignition circuit for a discharge lamp provided with preheatable electrodes comprising, two input terminals adapted to be connected to an alternating voltage supply source, a stabilizing ballast inductor, means connecting the series arrangement of the stabilizing ballast inductor and the lamp across said input terminals, a starter circuit for the discharge lamp comprising, a transistor, means connecting the transistor to the ends of the lamp preheatable electrodes remote from the input terminals, means connecting the base of the transistor to a control circuit having means for switching the transistor alternately conducting and nonconducting at a frequency substantially higher than the frequency of the AC voltage source during the ignition procedure of the lamp, a further electrical switching element connected in a branch circuit arranged in parallel with the transistor and provided with an operating member by means of which said switching element is triggered into a conducting condition thereby suppressing the switching operation of the transistor during the ignition procedure at a time no later than during the first half of each half cycle of said AC supply voltage, said further switching element remaining conductive for the remainder of each half cycle during the ignition procedure. A

2. An ignition circuit as claimed in claim 1 wherein the operating member of the further electrical switching element is chosen so that per half cycle of the AC voltage supply the ratio between the period that the transistor is being switched and the total period of the half cycle is in the range between 0.01 and 0.5.

3. An ignition circuit as claimed in claim 1 wherein the further electrical switching element comprises a controlled semiconductor element and said operating member includes a glow discharge tube connected to the control electrode of the conrolled semiconductor element.

4. An ignition circuit as claimed in claim 1 further comprising a rectifier bridge, means connecting the ends of the preheatable electrodes remote from the input terminals to the transistor by means of said rectifier bridge, means connecting a control circuit having a short time constant to two output terminals of said rectifier bridge, means connecting the base of the transistor to said control circuit through a breakdown element, characterized in that the further electrical switching element comprises a semiconductor switching element having a first main electrode and a control electrode connected to one output terminal of the rectifier bridge in a manner such that the main electrode is connected directly thereto and the control electrode is connected thereto through a resistor, and means connecting the other main electrode of the semiconductor switching element to the other output terminal of the rectifier bridge.

5. An ignition circuit as claimed in claim 4, characterized in that the further electrical switching element is a thyristor.

6. An electric supply circuit for an electric discharge lamp having a pair of preheatable electrodes comprising, a pair of input terminals for applying an AC supply voltage of a given frequency to the supply circuit, a ballast impedance, means connecting the ballast impedance in series with the discharge lamp across said input terminals, a combination heating and ignition circuit for the discharge lamp comprising, a first controlled semiconductor switch, means connecting said first semiconductor switch to the lamp electrodes to form a heating circuit for the electrodes that allows a heating current to flow from the input terminals to the lamp electrodes when said first semiconductor switch is turned on, a control circuit energizable from said input terminals and coupled to the control electrode of said first semiconductor switch and including means for alternately triggering the first semiconductor switch on and off at a frequency substantially higher than said AC given frequency and during the early part of each half cycle of said AC supply voltage, a second controlled semiconductor switch connected in a branch circuit across said first semiconductor switch for controlling the switching operation of the first semiconductor switch by effectively overiding said control circuit triggering means, and actuating means coupled to the control electrode of the second semiconductor switch for controlling the operation thereof whereby the switching operation of the first semiconductor switch is terminated at a time no later than during the first half of each half cycle of said AC supply voltage.

7. A supply circuit as claimed in claim 6 wherein said actuating means is chosen to trigger the second semiconductor switch at a time in each half cycle of the AC supply voltage such that the switching period of the first semiconductor switch is between and 1/5 of a half period of said AC voltage.

8. A supply circuit as claimed in claim 6 further comprising current rectifying means connected between the lamp electrodes and the combination heating and ignition circuit whereby the combination circuit is energized with a DC voltage but the lamp electrodes are heated with an AC current, and wherein said actuating means triggers said second semiconductor switch into conduction for terminating the switching operation of the first semiconductor switch and said second semiconductor switch continues to conduct substantially for the remaining part of each half cycle of the AC supply voltage subsequent to the trigger time thereby providing a heating current path for the lamp electrodes through the second semiconductor switch.

9. A supply circuit as claimed in claim 7 further comprising current rectifying means connected between the lamp electrodes and the combination heating and ignition circuit whereby the combination circuit is energized with a DC voltage but the lamp electrodes are heated with an AC current, and wherein said actuating means triggers said second semiconductor switch into conduction for terminating the switching operation of the first semiconductor switch and said second semiconductor switch continues to conduct substantially for the remaining part of each half cycle of the AC supply voltage subsequent to the trigger time, and said second semiconductor switch is coupled to the control electrode of the first semiconductor switch whereby conduction in the second switch holds the first switch in conduction to provide a heating current path for the lamp electrodes switch.

10. A supply circuit as claimed in claim 6 wherein said actuating means triggers said second semiconductor switch into conduction for terminating the switching operation of the first semiconductor switch and said second semiconductor switch continues to conduct substantially for the remaining part of each half cycle of the AC supply voltage subsequent to the trigger time.

11. A supply circuit as claimed in claim 10 wherein said control circuit triggering means includes a timing circuit that determines the alternate triggering of said first semiconductor switch and is chosen to trigger the first semiconductor switch at some time during the first half of each half cycle of the AC supply voltage.

12. A supply circuit as claimed in claim 6 further through the first semiconductor comprising current rectifying means connected between the lamp electrodes and the combination heating and ignition circuit whereby the combination circuit is energized with a DC voltage but the lamp electrodes are heated with an AC current.

13. A supply circuit as claimed in claim 8 wherein said control circuit includes an RC timing circuit having a short time constant that determines the alternate triggering of said first semiconductor switch. 

1. An ignition circuit for a discharge lamp provided with preheatable electrodes comprising, two input terminals adapted to be connected to an alternating voltage supply source, a stabilizing ballast inductor, means connecting the series arrangement of the stabilizing ballast inductor and the lamp across said input terminals, a starter circuit for the discharge lamp comprising, a transistor, means connecting the transistor to the ends of the lamp preheatable electrodes remote from the input terminals, means connecting the base of the transistor to a control circuit having means for switching the transistor alternately conducting and non-conducting at a frequency substantially higher than the frequency of the AC voltage source during the ignition procedure of the lamp, a further electrical switching element connected in a branch circuit arranged in parallel with the transistor and provided with an operating member by means of which said switching element is triggered into a conducting condition thereby suppressing the switching operation of the transistor during the ignition procedure at a time no later than during the first half of each half cycle of said AC supply voltage, said further switching element remaining conductive for the remainder of each half cycle during the ignition procedure.
 2. An ignition circuit as claimed in claim 1 wherein the operating member of the further electrical switching element is chosen so that per half cycle of the AC voltage supply the ratio between the period that the transistor is being switched and the total period of the half cycle is in the range between 0.0l and 0.5.
 3. An ignition circuit as claimed in claim 1 wherein the further electrical switching element comprises a controlled semiconductor element and said operating member includes a glow discharge tube connected to the control electrode of the conrolled semiconductor element.
 4. An ignition circuit as claimed in claim 1 further comprising a rectifier bridge, means connecting the ends of the preheatable electrodes remote from the input terminals to the transistor by means of said rectifier bridge, means connecting a control circuit having a short time constant to two output terminals of said rectifier bridge, means connecting the base of the transistor to said control circuit through a breakdown element, characterized in that the further electrical switching element comprises a semiconductor switching element having a first main electrode and a control electrode connected to one output terminal of the rectifier bridge in a manner such that the main electrode is connected directly thereto and the control electrode is connected thereto through a resistor, and means connecting the other main electrode of the semiconductor switching element to the other output terminal of the rectifier bridge.
 5. An ignition circuit as claimed in claim 4, characterized in that the further electrical switching element is a thyristor.
 6. An electric supply circuit for an electric discharge lamp having a pair of preheatable electrodes comprising, a pair of inpuT terminals for applying an AC supply voltage of a given frequency to the supply circuit, a ballast impedance, means connecting the ballast impedance in series with the discharge lamp across said input terminals, a combination heating and ignition circuit for the discharge lamp comprising, a first controlled semiconductor switch, means connecting said first semiconductor switch to the lamp electrodes to form a heating circuit for the electrodes that allows a heating current to flow from the input terminals to the lamp electrodes when said first semiconductor switch is turned on, a control circuit energizable from said input terminals and coupled to the control electrode of said first semiconductor switch and including means for alternately triggering the first semiconductor switch on and off at a frequency substantially higher than said AC given frequency and during the early part of each half cycle of said AC supply voltage, a second controlled semiconductor switch connected in a branch circuit across said first semiconductor switch for controlling the switching operation of the first semiconductor switch by effectively overiding said control circuit triggering means, and actuating means coupled to the control electrode of the second semiconductor switch for controlling the operation thereof whereby the switching operation of the first semiconductor switch is terminated at a time no later than during the first half of each half cycle of said AC supply voltage.
 7. A supply circuit as claimed in claim 6 wherein said actuating means is chosen to trigger the second semiconductor switch at a time in each half cycle of the AC supply voltage such that the switching period of the first semiconductor switch is between 1/20 and 1/5 of a half period of said AC voltage.
 8. A supply circuit as claimed in claim 6 further comprising current rectifying means connected between the lamp electrodes and the combination heating and ignition circuit whereby the combination circuit is energized with a DC voltage but the lamp electrodes are heated with an AC current, and wherein said actuating means triggers said second semiconductor switch into conduction for terminating the switching operation of the first semiconductor switch and said second semiconductor switch continues to conduct substantially for the remaining part of each half cycle of the AC supply voltage subsequent to the trigger time thereby providing a heating current path for the lamp electrodes through the second semiconductor switch.
 9. A supply circuit as claimed in claim 7 further comprising current rectifying means connected between the lamp electrodes and the combination heating and ignition circuit whereby the combination circuit is energized with a DC voltage but the lamp electrodes are heated with an AC current, and wherein said actuating means triggers said second semiconductor switch into conduction for terminating the switching operation of the first semiconductor switch and said second semiconductor switch continues to conduct substantially for the remaining part of each half cycle of the AC supply voltage subsequent to the trigger time, and said second semiconductor switch is coupled to the control electrode of the first semiconductor switch whereby conduction in the second switch holds the first switch in conduction to provide a heating current path for the lamp electrodes through the first semiconductor switch.
 10. A supply circuit as claimed in claim 6 wherein said actuating means triggers said second semiconductor switch into conduction for terminating the switching operation of the first semiconductor switch and said second semiconductor switch continues to conduct substantially for the remaining part of each half cycle of the AC supply voltage subsequent to the trigger time.
 11. A supply circuit as claimed in claim 10 wherein said control circuit triggering means includes a timing circuit that determines the alternate triggering of said first semiconductor switch and iS chosen to trigger the first semiconductor switch at some time during the first half of each half cycle of the AC supply voltage.
 12. A supply circuit as claimed in claim 6 further comprising current rectifying means connected between the lamp electrodes and the combination heating and ignition circuit whereby the combination circuit is energized with a DC voltage but the lamp electrodes are heated with an AC current.
 13. A supply circuit as claimed in claim 8 wherein said control circuit includes an RC timing circuit having a short time constant that determines the alternate triggering of said first semiconductor switch. 